The present invention relates to an information recording medium for recording information on a magnetic layer in a corresponding direction of magnetization.
Hard disk devices (HD) and magneto-optical disk devices (MO) are the mainly used devices among recording/reading devices comprising an information recording medium. A HD comprises a magnetic medium having a magnetic layer of in-plane magnetic anisotropy. The HD also includes a magnetic head having a magnetic field generating unit for recording and a sensor unit for reading. The magnetic head is disposed as to be opposing a face of the magnetic medium for recording a recording bit in the magnetic medium by changing the direction of magnetic field of the magnetic head while rotating the magnetic medium. The recording bit stores therein a direction of magnetization in an in-plane direction which varies in correspondence with the information, and reading the information is performed by detecting an amplitude of a readout signal with the sensor unit for reading. Presently, an information recording density of 1 Gbit/inch.sup.2 is realized by using a thin-film sensor.
On the other hand, an MO comprises a magneto-optical medium having a magnetic layer of perpendicular magnetic anisotropy. The MO also includes a sensor unit for emitting laser beams to the medium and receiving the reflections thereof, and a magnetic field generating unit. A recording bit is recorded in the magneto-optical medium by emitting laser beams and applying a magnetic field while rotating the magneto-optical medium. The recording bit stores therein a direction of magnetization in a perpendicular direction which varies in correspondence with the information. To read the information, by a rotating direction of a plane of polarization from the received reflected light of the laser beam is defected. By using a recently developed magnetically-induced super resolution (MSR) technique, an information recording density of about 3 Gbit/inch.sup.2 is realized.
In order to further improve the recording density of media in such HDs and MOs as explained above, the size of the bit of the media, i.e. the bit length in a tangential direction and the bit width in a radial direction, needs to be reduced.
In order to realize a recording density of 1.5 Gbit/inch.sup.2 in a HD, a track pitch of approximately 2.5 .mu.m and a minimum bit length of approximately 0.14 .mu.m is required. Likewise, the track pitch and minimum bit length need to be approximately 1.25 .mu.m and 0.09 .mu.m, respectively, if a recording density of 5 Gbit/inch.sup.2 is to be realized. In the case of an MO, a recording density of 1 Gbit/inch.sup.2 is already realized with a track pitch of 1.1 .mu.m and minimum bit length of approximately 0.48 .mu.m. In order to realize a recording density of 3.5 Gbit/inch.sup.2, the track pitch and minimum bit length need to be 0.7 .mu.m and approximately 0.27 .mu.m, respectively.
It is obvious from the above explanations that in obtaining a high recording density of the media of HDs and MOs, a HD has an advantage in that the minimum bit length thereof can be made shorter than a minimum bit length of an MO by the use of a MR head, but is disadvantaged in that the track pitch is broader. It is difficult to provide a track pitch of 1 .mu.m or less due to problems in tracking techniques and head processing techniques. An MO has an advantage in that the track pitch is narrower than a track pitch in a HD, but disadvantaged in that the minimum bit length is longer and its resolution can not be made to be 0.1 .mu.m or less as it is possible by using a MR head, even by using MSR techniques. By specifying the magnetic layer of the magneto-optical medium in an MO, it is possible to read a recording bit of a minimum bit length of approximately 0.1 .mu.m, but such a case presents a drawback in that a C/N of a magneto-optical readout signal tends to be low.